1. Field of the Invention
The present invention relates in general to opening and closing of a camcorder iris diaphragm, and more particularly to a method and circuit for controlling the opening and closing of the camcorder iris diaphragm in accordance with a magnitude of a luminance signal received therein.
2. Description of the Prior Art
Typically, an iris diaphragm is mounted in a camera for taking a picture of an object, such as a still camera, a movie camera, a camcorder (camera and recorder) and the like. In taking a picture of an object, it is necessary to control the opening and closing of the camcorder iris diaphragm in accordance with a magnitude of a luminance signal received therein. In other words, the larger the magnitude of the luminance signal becomes, the smaller the opening level of the iris diaphragm must be; alternatively, the smaller the magnitude of the luminance signal becomes, the larger the opening level of the iris diaphragm must be.
Referring to FIG. 1, there is shown a block diagram of a circuit for controlling opening and closing of a camcorder iris diaphragm in accordance with the prior art. The illustrated circuit comprises a luminance signal detector 1 for detecting a luminance signal Y from an image signal of an object received therein, a buffer 2 for buffering the luminance signal Y output from the luminance signal detector 1, a signal converter 3 for converting the luminance signal Y output from the buffer 2 into a direct current (DC) signal of the mean level value of the luminance signal Y, a differential amplifier 4 for amplifying a level difference between the DC signal from the signal converter 3 and a reference DC signal of a predetermined level, an iris motor 6, and a motor driver 5 for driving the iris motor 6 in accordance with a level difference between an output signal from the differential amplifier 4 and another reference DC signal.
The differential amplifier 4 includes a variable resistor VR1 for varying a predetermined DC voltage Vcc, an operational amplifier OP1 having its inverting input terminal (-) for inputting the reference DC signal, or an output signal from the variable resistor VR1 and its non-inverting input terminal (+) for inputting the converted DC luminance signal, that is, the output signal from the signal converter 3, and parallel connected capacitor C1 and resistor R1 connected between an output terminal of the operational amplifier OP1 and the inverting input terminal (-) thereof.
The operation of the conventional circuit with the above-mentioned construction will now be described.
First, the luminance signal detector 1 detects the luminance signal Y from the image signal which includes the luminance signal Y and a chrominance signal C. Then, the detected luminance signal Y from the detector 1 is buffered by the buffer 2 which then applies the buffered luminance signal Y to the signal converter 3. Upon receiving the luminance signal Y from the buffer 2, the signal converter 3 converts the received luminance signal Y into a DC signal of the mean level value of the luminance signal Y and applies the DC signal to the non-inverting input terminal (+) of the operational amplifier OP1 included in the differential amplifier 4, the inverting input terminal (-) of which is applied with a reference DC signal of a predetermined level given by the variable resistor VR1. As a result, the operational amplifier OP1 in the differential amplifier 4 operates to amplify a level difference between the DC signal at its non-inverting input terminal (+) and the reference DC signal at its inverting input terminal (-). Then, the output signal from the operational amplifier OP1 is applied as a negative (-) drive signal to the motor driver 5. Motor driver 5 is also connected to another reference DC signal, predetermined positive (+) drive signal +V.sub.CM. In accordance with a level difference between the negative (-) drive signal and the positive (+) drive signal +V.sub.CM, the motor driver 5 drives the iris motor 6, thereby allowing the iris diaphragm (not shown) to be opened and closed according to the rotational direction and speed of the iris motor 6.
Namely, the level of the DC signal from the signal converter 3 is increased as the luminance signal Y component of the image signal is increased based on circumstances of a picture of an object being taken. Because of an increase in the level of the input signal to the non-inverting input terminal (+), subsequently, the operational amplifier OP1 in the differential amplifier 4 outputs the level difference increased signal. Since such level difference increased signal from the operational amplifier OP1 is applied as the negative (-) drive signal to the motor driver 5, the level difference between the negative (-) drive signal and the positive (+) drive signal +V.sub.CM becomes relatively smaller. For this reason, the motor driver 5 operates to drive the iris motor 6 in a direction to close the iris diaphragm.
On the other hand, as the luminance signal Y component of the image signal is decreased based on circumstances of a picture of an object being taken, the level of the DC signal from the signal converter 3 is decreased and, therefore, the operational amplifier OP1 in the differential amplifier 4 outputs a decreased level difference signal corresponding to a decrease in the level of the input signal to the non-inverting input terminal (+). Since such level difference decreased signal from the operational amplifier OP1 is applied as the negative (-) drive signal to the motor driver 5, the level difference between the negative (-) drive signal and the positive (+) drive signal +V.sub.CM becomes relatively larger. Consequently, the motor driver 5 operates to drive the iris motor 6 in a direction opening the iris diaphragm.
However, the above-mentioned conventional circuit has the following disadvantage. That is, in a case where an object to be taken is a light source of high brightness, such as a solar light, the luminance signal Y component is thus increased, resulting in an increase in the level of the DC signal from the signal converter 3. Hence, the operational amplifier OP1 in the differential amplifier 4 outputs a corresponding increase level difference signal, thereby causing the level difference between the negative (-) drive signal and the positive (+) drive signal +V.sub.CM to become relatively smaller. As a result, the motor driver 5 operates to drive the iris motor 6 in a direction closing the iris diaphragm. Ultimately, the iris diaphragm is fully closed because the object being taken is the light source of high brightness, such as a solar light. Since an incoming light through a lens (not shown) is blocked by the full closing of the iris diaphragm, the luminance signal Y becomes a low signal level representative of no signal. At this time, the level of the DC signal from the signal converter 3 correspondingly falls to zero level and, therefore, the operational amplifier OP1 in the differential amplifier 4 outputs a decreased level difference signal, thereby causing the level difference between the negative (-) drive signal and the positive (+) drive signal +V.sub.CM to become relatively larger. As a result, the motor driver 5 operates to drive the iris motor 6 in a direction opening the iris diaphragm. In other words, in a situation where a picture of an object taken is a light source of high brightness, such as a solar light, the conventional circuit repeatedly performs temporarily closing the iris diaphragm completely secure of the high brightness, then opens the iris diaphragm because no luminance signal caused by the full closing of the iris diaphragm. As a result, the opening and closing of the iris diaphragm is oscillatingly repeated at a voltage level V.sub.DC (close) as shown in FIG. 2. For this reason, the camcorder with the above-mentioned conventional circuit cannot take a picture of an object such as a light source of high brightness (for example, a solar light), without a separate mechanism such as a neutral density (ND) filter.